1. Field of the Invention
This invention relates generally to a method for forming a p-channel semiconductor device in which the threshold voltage is not a function of channel length, and more particularly to a method which utilizes boron penetration to suppress short channel effect.
2. Brief Description of the Prior Art
Metal oxide semiconductor (MOS) devices are well known in the prior art. FIG. 1 illustrates a p-type MOS device. The device 10 includes an n-doped silicon layer 12, and p-doped silicon source 14 and drain 16 regions. Note that the n-doped silicon substrate could also be an n-well or n-tub inside a p-substrate. The separation between the drain and source regions is typically around 0.8 .mu.m in the present state of the art. A layer of silicon dioxide 20 overlies these areas, and a polysilicon layer 22 is placed over the area ("channel") between source and drain regions, defining a "gate" region. Normally with OV or positive voltage on the gate, no p-type conduction is formed between the source and drain regions, thus no current can flow between them. When a negative bias is applied across the gate region, the n-type region in channel 18 will gradually be depleted away until it becomes "inverted", i.e., a p-type conduction region is formed out of the n-type region that connects source and drain regions 14 and 16. Current can then flow from source region to drain region through the inversion layer in channel region 18. FIG. 2(a) illustrates that for a fixed channel length (typically on the order of 0.8 .mu.m), the current from the source to the drain region will approach a constant value known as saturation current when the gate voltage is increasing in the negative direction and reaches the threshold level (V.sub.t) where current flow reaches a level suitable for reliable operation of the device.
FIG. 2(b) illustrates current flow from source to drain regions with different channel lengths. Note that at shorter channel lengths, current flows even at positive voltages, and reliable device performance (which requires that the device be distinctly "on" or "off") cannot be obtained.
FIG. 3 illustrates that threshold voltage is dependent on the length of the p-channel transistor. In order to obtain satisfactory performance, the channel length used must be at least as great as that required to reach the relatively flat portion of the curve shown in FIG. 3. Since device size is limited by the channel length, reduction of the channel length is desired to obtain devices with smaller geometry. The "short channel" effect shown in FIG. 3 (i.e. variation in threshold voltage at short channel lengths) limits device geometry. Typically, additional n-doping could be added to raise the threshold voltage (V.sub.T) of short channel transistors (i.e. 0.7 .mu.m or less). However, such doping also affects longer channel transistors. Hence, the V.sub.T for these longer channels is higher than desirable because the higher V.sub.T will render low current drive and consequently the devices will be slower.